The present invention relates to a pipe coupling having a dynamic axial restraint to system for anchoring pipes to lock them axially together. The invention finds particular application in the connection of plastic pipes.
A wide variety of pipe couplings for connecting together two pipe ends in a fluid tight manner are known. The fluid pressure in a pipeline tends to force the pipe ends apart and known pipe couplings incorporate an arrangement for securing the pipes against axial movement relative to the coupling upon axial loading of the pipes.
For example, EP-A-0 542 779 discloses a pipe coupling incorporating a frusto-conical gripping ring with inwardly projecting gripping teeth, which is located within a circumferential slot provided in the outer surface of the sealing gasket. In use, when the coupling is tightened around the pipe, the gripping teeth penetrate the sealing gasket at the bottom of the slot and engage the outer surface of the pipe to hold the pipe ends against axial displacement. As the axial load on the pipe increases, the frusto-conical gripping ring tends to flatten, causing the teeth to bite deeper into the pipe surface, improving the grip.
The sustained use of this type of axial restraint system including a gripping ring may additionally cause some degradation of the pipe, due to the gripping teeth cutting into the pipe surface. This is not problematic for metallic pipes but in the case of softer metal or plastic pipes, it has been found that such a system can only withstand relatively moderate levels of axial loading over a long period, in particular when a plastic pipe is subjected to variations in temperature.
Furthermore, at high levels of axial loading, the gripping ring will tend to rotate away from the outer surface of the pipe, which reduces the level of grip. Eventually, the gripping ring may flip over and lose contact with the pipe entirely. Where the pipe is formed of a relatively thin material, the rotation of the gripping ring relative to the pipe surface may additionally cause deformation of the cross-sectional shape of the pipe end.
There is a strong demand for plastic pipes for high pressure applications. However, it has been observed that some thermoplastic materials used for forming pipes will deform upon the application of stress and over time, the plastic will tend to move to a point of lower stress or resistance. This means that in the region of a pipe coupling, the prolonged application of radial pressure by the gripping ring clamped around the pipe will cause the plastic to migrate or ‘creep’ away from the gripping teeth to an adjacent area of lower stress. As the plastic migrates away from the gripping ring, the gripping teeth are unable to retain a sufficiently strong grip on the pipe surface.
It would therefore be desirable to provide a novel pipe coupling having an axial restraint system that provides improved gripping of the pipe surface and improved resistance to axial movement of the pipe ends. It would be particularly desirable to provide such a novel pipe coupling that provides improved axial restraint for plastic pipes and that performs better than known pipe couplings under high axial loads.